Gallium arsenide avalanche photodiode

ABSTRACT

An avalanche photodiode can include: an avalanche region having one or more layers prepared from GaAs; an N −  absorption layer extending across the avalanche region; an N-type layer above at least a center portion of the N −  absorption layer; and optionally a lower conductivity layer laterally from the N-type layer to a surface of the avalanche region and above a perimeter portion of the N −  absorption layer, the lower conductivity layer having lower conductivity compared to the N-type layer. The avalanche photodiode can include a window layer above the N-type layer and lower conductivity layer, and an anode contact above the window layer. The avalanche photodiode can include an N +  barrier layer below the N −  absorption layer, an N +  conduction layer below the N +  barrier layer, a substrate below the N +  conduction layer, and a cathode contact coupled with the N +  conduction layer.

CROSS-REFERENCE

This U.S. patent application claims the benefit of U.S. provisionalpatent application having Ser. No. 61/878,699 filed Sep. 17, 2013, whichprovisional application is incorporated herein by specific reference inits entirety.

BACKGROUND

Communication modules, such as electronic or optoelectronic transceiversor transponder modules, are used in electronic and optoelectroniccommunication. Such communication modules may include multiplecomponents that operate at various voltages. An example of such acomponent is an optical receiver that converts an optical signal(incident light) into an electrical signal or voltage using, forexample, a photodetector. A photodiode or photodiode array may be usedas the photodetector to generate the electrical signal from the incidentlight. The communication module may also include an electrical circuitto receive and process the electrical signal from the photodetector tosuit the desired application.

One very sensitive type of photodiode is an avalanche photodiode.Avalanche photodiodes are well-known devices that serve at least twofunctions: 1) conversion of optical signals into electrical signals; and2) amplification of the electrical signal through avalanchemultiplication. Typically, an avalanche photodiode has an absorptionlayer where an optical signal is absorbed. Photons in the optical signalimpinging the absorption layer generate an electron-hole pair or acarrier pair. A multiplication layer in the avalanche photodiode isdesigned such that one carrier causes an avalanche of other carrierswhere the number of other carriers is dependent on the gain of theavalanche photodiode.

The subject matter claimed herein is not limited to embodiments thatsolve any disadvantages or that operate only in environments such asthose described above. Rather, this background is only provided toillustrate one example technology area where some embodiments describedherein may be practiced.

SUMMARY

In one embodiment, an avalanche photodiode can include: an avalancheregion having one or more layers prepared from GaAs. The avalanchephotodiode can include: an N⁻ absorption layer extending across theavalanche region; an N-type layer above at least a center portion of theN⁻ absorption layer; and optionally a lower conductivity layer laterallyfrom the N-type layer to a surface of the avalanche region and above aperimeter portion of the N⁻ absorption layer, wherein the lowerconductivity layer has lower conductivity compared to the N-type layer.In one aspect, when included, the lower conductivity layer can have aconductivity reducing implant compared to conductivity of the N-typelayer. In one aspect, the implant is a proton implant or a berylliumimplant.

In one embodiment, the avalanche photodiode can include: an N⁻absorption layer extending across the avalanche region; and an N-typelayer above the N⁻ absorption layer and extending across the avalancheregion.

In one embodiment, the avalanche photodiode can include: a N⁻ absorptionlayer extending all the way across a mesa of the avalanche region sothat the edges of the N⁻ absorption layer form part of the sloped sidewall of the mesa; and a N-type layer above the N⁻ absorption layerextending all the way across the mesa of the avalanche region so thatthe edges of the N-type layer form part of the sloped side wall of themesa. Accordingly, the N-type layer may not have any material or regionlaterally thereof in the mesa.

In one embodiment, the avalanche photodiode can include a transparentP-type window layer above the N-type layer and lower conductivity layer,and an anode contact above the window layer.

In one embodiment, the avalanche photodiode can include: a transparentN⁺ barrier layer below the N⁻ absorption layer, an N⁺ conduction layerbelow the N⁺ barrier layer, a substrate below the N⁺ conduction layer,and a cathode contact coupled with the N⁺ conduction layer. Here,orientation includes the top receiving the light and the bottom beingthe substrate.

In one embodiment, the avalanche region is included in a mesa structure,the mesa extending into the N⁺ conduction layer so that a portion of theN⁺ conduction layer is in the mesa and a portion forms a shoulder fromthe mesa, and with the cathode contact on the shoulder of the N⁺conduction layer.

In one embodiment, the avalanche photodiode can include: an oxide layerabove the window layer; a dielectric passivation layer coated onto thewindow layer and over side surfaces of the mesa; an anode bond padcoupled with the anode contact; a cathode bond pad coupled with thecathode contact; and an environmental-protection passivation layer abovethe dielectric passivation layer, anode contact, and cathode contact,and with the environmental-protection passivation layer having openingsthat expose the anode bond pad and cathode bond pad.

In one embodiment, the windows (e.g., P⁺ window layers) and blockinglayers (e.g., N⁺ barrier layers) of the APD can be transparent, such asbeing transparent to the intended wavelength of operation of the APD.

In one embodiment, an active thickness of the APD can include the GaAslayers between the window layer (e.g., P⁺ window layer) and the blockinglayer (e.g., N⁺ barrier layer). The active thickness can be fullydepleted at the operating avalanche voltage.

In one embodiment, an avalanche photodiode can include: an avalancheregion having one or more layers prepared from GaAs. The avalanchephotodiode can include: a N⁻ absorption layer extending across theavalanche region; a N-type layer above a center portion of the N⁻absorption layer; and a lower conductivity layer laterally from theN-type layer to a surface of the avalanche region and above a perimeterportion of the N⁻ absorption layer, the lower conductivity layer havinglower conductivity compared to the N-type layer. The avalanchephotodiode can include a transparent P-type window layer above theN-type layer and above the N⁻ absorption layer, the window layerincluding the lower conductivity layer, and an anode contact above thewindow layer. The avalanche photodiode can include a transparent N⁺barrier layer below the N⁻ absorption layer, an N⁺ conduction layerbelow the N⁺ barrier layer, a substrate below the N⁺ conduction layer,and a cathode contact coupled with the N⁺ conduction layer. Theavalanche photodiode can include an avalanche region that is included ina mesa structure, the mesa extending into the N⁺ conduction layer sothat a portion of the N⁺ conduction layer is in the mesa and a portionforms a shoulder from the mesa, with the cathode contact on the shoulderof the N⁺ conduction layer. In one aspect, the avalanche photodiode caninclude: an oxide layer above the window layer; a dielectric passivationlayer coated onto the window layer and over side surfaces of the mesa;an anode bond pad coupled with the anode contact; a cathode bond padcoupled with the cathode contact; and an environmental-protectionpassivation layer above the dielectric passivation layer, anode contact,and cathode contact, and the environmental-protection passivation layerhaving openings that expose the anode bond pad and cathode bond pad.

In one embodiment, an avalanche photodiode can include: an avalancheregion having one or more layers prepared from GaAs; a P⁻ absorptionlayer extending across the avalanche region; a P-type layer below acenter portion of the P⁻ absorption layer; and a lower conductivityregion laterally from the P-type layer to a surface of the avalancheregion and below a perimeter portion of the P⁻ absorption layer, thelower conductivity region having lower conductivity compared to theP-type layer. In one aspect, the avalanche photodiode can include atransparent P-type window layer above the P⁻ absorption layer, the P⁻absorption layer including the lower conductivity region, and an anodecontact above the window layer. In one aspect, the avalanche photodiodecan include a transparent N⁺ barrier layer below the P⁻ absorptionlayer, an N⁺ conduction layer below the N⁺ barrier layer, a substratebelow the N⁺ conduction layer, and a cathode contact coupled with the N⁺conduction layer. In one aspect, the avalanche region is included in amesa structure, the mesa extending into the N⁺ conduction layer so thata portion of the N⁺ conduction layer is in the mesa and a portion formsa shoulder from the mesa, with the cathode contact on the shoulder ofthe N⁺ conduction layer. In one aspect, the avalanche photodiode caninclude: an oxide layer above the window layer; a dielectric passivationlayer coated onto the window layer and over side surfaces of the mesa;an anode bond pad coupled with the anode contact; a cathode bond padcoupled with the cathode contact; and an environmental-protectionpassivation layer above the dielectric passivation layer, anode contact,and cathode contact, and the environmental-protection passivation layerhaving openings that expose the anode bond pad and cathode bond pad.

In one embodiment, a method of manufacturing an avalanche photodiode caninclude: growing a base epitaxial structure having a substrate, aconduction layer above the substrate, a barrier layer above theconduction layer, an absorption layer above the barrier layer, and anactive layer (e.g., N-type layer GaAs) above the absorption layer, theabsorption layer and active layer having GaAs with different doping;processing the active layer to remove an annular perimeter region toleave a center active region; and growing a window layer over the centeractive region of the active layer and over the absorption layer. In oneaspect, the active layer produced by this method is an N-type activelayer.

In one embodiment, a method of manufacturing an avalanche photodiode caninclude: growing a base epitaxial structure having a substrate, aconduction layer above the substrate, a barrier layer above theconduction layer, an absorption layer above the barrier layer, and anactive layer (e.g., N-type layer GaAs) above the absorption layer, theabsorption layer and active layer having GaAs with different doping; andgrowing a window layer over the active layer and over the absorptionlayer. In one aspect, the active layer produced by this method is anN-type active layer.

In one embodiment, a method of manufacturing an avalanche photodiode caninclude: growing a base epitaxial structure having a substrate, aconduction layer above the substrate, a barrier layer above theconduction layer, an active layer (e.g., P-type layer P⁺ GaAs) above thebarrier layer where the active layer is etched at the annular perimeterregion to leave a central region, and an absorption layer (e.g., P⁻absorption layer) above the barrier layer at the annular perimeterregion where the active layer was etched away; and growing a windowlayer over the center active region of the active layer and over theabsorption layer. In one aspect, the active layer is a P-type activelayer.

In one embodiment, a method of manufacturing an avalanche photodiode caninclude: growing a base epitaxial structure having a substrate, aconduction layer above the substrate, a barrier layer above theconduction layer, an active layer (e.g., P-type layer P⁺ GaAs) above thebarrier layer, and an absorption layer (e.g., P⁻ absorption layer) abovethe barrier layer; and growing a window layer over the center activeregion of the active layer and over the absorption layer. In one aspect,the active layer is a P-type active layer.

In one embodiment, when the active layers are N-type, the N⁻ portion ofthe active thickness can be adjacent to the N⁺ blocking layer, and thehigher conductivity N-type portion of the active thickness can beadjacent to the P⁺ window layer.

In one embodiment, when the active thickness is P-type, the P⁻ portionof the active thickness can be adjacent to the P⁺ window layer, and thehigher conductivity P-type portion of the active thickness can beadjacent to the N⁺ barrier layer.

In one embodiment, the method includes forming the window layer to havea center stepped-up region above the center active region of the activelayer compared to a perimeter stepped-down region above the absorptionlayer, wherein the center stepped-up region and perimeter stepped-downregion includes about the same thickness of material.

In one embodiment, a method of manufacturing an avalanche photodiode caninclude: growing a base epitaxial structure having a substrate, aconduction layer above the substrate, a barrier layer above thesubstrate, an absorption layer above the substrate, and an active layerabove the absorption layer, absorption layer, and active layer havingGaAs with different doping; processing the active layer withimplantation to reduce conductivity of an annular perimeter region toleave a center active region; and growing a window layer over the centeractive region of the active layer and over the absorption layer. In oneaspect, the implantation is proton or beryllium implantation.

In one embodiment, any of the manufacture methods can include thefollowing: forming an oxide layer over the window layer; etching thebase epitaxial structure to form a mesa structure; depositing adielectric passivation layer over the mesa; etching the dielectricpassivation layer to form an anode contact area and a cathode contactarea; forming an anode contact in the anode contact area; forming acathode in the cathode contact area; forming an anode bond pad that iselectronically coupled with the anode contact area; forming a cathodebond pad that is electronically coupled with the cathode contact area;depositing an environmental-protection passivation layer; and removingportions of the environmental-protection passivation layer to expose theanode bond pad and cathode bond pad.

The foregoing summary is illustrative only and is not intended to be inany way limiting. In addition to the illustrative aspects, embodiments,and features described above, further aspects, embodiments, and featureswill become apparent by reference to the drawings and the followingdetailed description.

BRIEF DESCRIPTION OF THE FIGURES

The foregoing and following information as well as other features ofthis disclosure will become more fully apparent from the followingdescription and appended claims, taken in conjunction with theaccompanying drawings. Understanding that these drawings depict onlyseveral embodiments in accordance with the disclosure and are,therefore, not to be considered limiting of its scope, the disclosurewill be described with additional specificity and detail through use ofthe accompanying drawings.

FIG. 1 includes an illustration of a transceiver.

FIG. 1A includes an illustration of a receiver having an array ofphotodiodes.

FIG. 2 includes an illustration of an embodiment of an avalanchephotodiode with an N-type active region formed by an interrupted growthprocess.

FIG. 3 includes an illustration of an embodiment of an avalanchephotodiode with an N-type active region having selective implantation toreduce conductivity in the implanted areas.

FIG. 3A includes an illustration of an embodiment of an avalanchephotodiode with an N-type active region having an N-type GaAs layerextending across the photodiode between slopes of a mesa.

FIG. 4 includes an illustration of an embodiment of an avalanchephotodiode with a P-type active region.

FIG. 4A includes an illustration of another embodiment of an avalanchephotodiode with a P-type active region.

FIG. 5 includes an illustration of an interrupted growth process forpreparing an avalanche photodiode.

FIG. 6 includes an illustration of a portion of a wafer having aplurality of avalanche photodiode chips.

FIG. 7 includes an illustration of an avalanche photodiode chip.

FIG. 8 includes a view of an intermediate avalanche photodiode duringmanufacture having an oxide layer.

FIG. 9 includes a view of an intermediate avalanche photodiode duringmanufacture having a passivation layer on the mesa.

FIG. 10 includes a view of an intermediate avalanche photodiode duringmanufacture having a passivation layer on the mesa, with the passivationlayer having etched openings for anode and cathode contact areas.

FIG. 11 includes a view of an intermediate avalanche photodiode duringmanufacture having a passivation layer on the mesa, with the passivationlayer having anode and cathode contacts therein.

FIG. 12 includes a compositional and doping diagram of an embodiment ofan avalanche photodiode.

FIG. 13 includes an E-field diagram of an embodiment of an avalanchephotodiode.

DETAILED DESCRIPTION

In the following detailed description, reference is made to theaccompanying drawings, which form a part hereof. In the drawings,similar symbols typically identify similar components, unless contextdictates otherwise. The illustrative embodiments described in thedetailed description, drawings, and claims are not meant to be limiting.Other embodiments may be utilized, and other changes may be made,without departing from the spirit or scope of the subject matterpresented herein. It will be readily understood that the aspects of thepresent disclosure, as generally described herein, and illustrated inthe figures, can be arranged, substituted, combined, separated, anddesigned in a wide variety of different configurations, all of which areexplicitly contemplated herein.

The terms and words used in the following description and claims are notlimited to the bibliographical meanings, but are merely used to enable aclear and consistent understanding of the disclosure. It is to beunderstood that the singular forms “a,” “an,” and “the” include pluralreferents unless the context clearly dictates otherwise. Thus, forexample, reference to “a component surface” includes reference to one ormore of such surfaces.

By the term “substantially” it is meant that the recited characteristic,parameter, or value need not be achieved exactly, but that deviations orvariations, including for example, tolerances, measurement error,measurement accuracy limitations and other factors known to thoseskilled in the art, may occur in amounts that do not preclude the effectthe characteristic was intended to provide.

In this disclosure the term “optoelectronic subassembly” may be used torefer to any portion of an optoelectronic assembly. However, at timesthis disclosure may use “optoelectronic subassembly” to refer tospecific portions of an optoelectronic assembly, as may be indicated bycontext.

High-speed fiber optic networks use light signals (which may also bereferred to as optical signals) to transmit data over a network. Fiberoptic networks have various advantages over other types of networks suchas copper wire based networks. Many existing copper wire networksoperate at near maximum possible data transmission rates and at nearmaximum possible distances for copper wire technology. Fiber opticnetworks are able to reliably transmit data at higher rates over furtherdistances than is possible with copper wire networks.

Although fiber optic networks use light signals to carry data, manyelectronic devices such as computers and other network devices useelectrical signals. Accordingly, optoelectronic assemblies may be usedto convert electrical signals to optical signals, convert opticalsignals to electrical signals, or convert both electrical signals tooptical signals and optical signals to electrical signals.

Optoelectronic assemblies may include optoelectronic subassemblies(“OSAs”), such as receiver optoelectronic subassemblies (“ROSAs”),transmitter optoelectronic subassemblies (“TOSAs”), or both. A ROSAreceives light signals with a light detector such as a photodiode andconverts light signals into electrical signals. A TOSA receiveselectrical signals and transmits corresponding light signals. A TOSA mayinclude an optical transmitter such as a laser that generates light thatis transmitted to a fiber optic network. Optoelectronic assemblies orsubassemblies may include various components such as optical componentsand/or electronic components.

Some optoelectronic assemblies may include multiple channels(“multi-channel optoelectronic assemblies”), with each channelcorresponding to a set of one or more optical signals travelling throughan optical fiber. Multi-channel optoelectronic assemblies may supportincreased data transfer rates through fiber optic networks. For example,a four channel optoelectronic assembly may be able to send and receivedata at data transfer rates of approximately four times the datatransfer rate of a comparable single channel optoelectronic assembly.

Ferrule assemblies may be used in fiber optic networks, such as at fiberports, to physically and/or optically couple optical fibers withoptoelectronic assemblies, optoelectronic subassemblies, opticalcomponents, and/or electronic components. For example, ferruleassemblies may be used to couple ROSAs and/or TOSAs to optical fibersthat are part of a fiber optic network thereby permitting the ROSA toreceive optical signals and/or permitting the TOSA to transmit opticalsignals. Additionally or alternatively, ferrule assemblies may form partof an optoelectronic assembly or subassembly configured to transmit orreceive electrical or optical signals in a fiber optic network.

Optoelectronic assemblies may need to comply with certain standards thatmay specify aspects of optoelectronic assemblies such as size, powerhandling, component interfaces, operating wavelengths, or otherspecifications. Examples of such standards include CFP, XAUI, QSFP,QSFP⁺, XFP, SFP, and GBIC. Complying with such standards may limit thestructure, size, cost, performance, or other aspects of optoelectronicassembly designs. Such standards may also limit configurations ofcomponents of optoelectronic assemblies such as receptacles that receiveferrule assemblies and/or hermetic sealing structures such as housings.

Generally, the present technology is related to avalanche photodiodesthat include gallium arsenide (GaAs) in the active region as well asoptoelectronic assemblies or subassemblies and ROSAs having thephotodiodes. The avalanche photodiodes can be used in 10G and 14G SRtransceivers, as well as others. The avalanche photodiodes can be usedin receivers and transceivers. The avalanche photodiodes can boostreceiver sensitivity by boosting photodiode responsivity via the GaAs.

FIG. 1 illustrates a transceiver 100 having a transmitter 102 and areceiver 104 as is common in transceiver communication modules. Thereceiver 104 can include a photodiode 106. An avalanche photodiode 106can be configured as any embodiment of an avalanche photodiode describedherein. Also, in some embodiments the transmitter 102 may include amonitor photodiode 108 to monitor light output from a light-emittingdevice 110, where the monitor photodiode 108 may be configured as anavalanche photodiode in accordance with one of the embodiments describedherein. The receiver 104 can include receiver circuitry 112 that canoperate with the photodiode 106 as well as other components common inreceivers 104. The transmitter 102 can include transmitter circuitry 114that can operate with the light-emitting device 110 and/or the monitorphotodiode 108 as well as other components common in transmitters 102.All components of the transmitter 102 and/or the receiver 104 of thetransceiver 100 can be operably coupled together as generally known andemployed in the art. Additionally, the transmitter 102 and the receiver104 can each include a fiber port 111 to receive an optical fiber. Thereceiver 104 can include the photodiode 106 having an anti-reflectionlayer 107 on the entrance that receives light from the fiber port 111.The anti-reflection layer 107 can be a dielectric layer on the surfaceof the photodiode 106, and it can be configured to inhibit or minimizefront surface reflection so that light is not reflected back into thefiber and transmitted back to the light emitter (e.g., laser diode,VCSEL, etc.) that is transmitting light to the photodiode 106.

Additionally, the photodiode of the invention can be used alone in areceiver for receiving optical communication signals. That is, areceiver can include a single photodiode, which can be used with onechannel. Also, a receiver can include multiple channels each with anoptical fiber optically coupled with a photodiode. Each channel caninclude its own optical fiber and photodiode. The optical fibers andphotodiodes can be arranged in parallel. FIG. 1A shows the receiver 104having “n” photodiodes 106 (e.g., along with fiber ports 111 and theanti-reflection layer 107) being an integer, such as 1, 2, 3, 4, 5, 6,7, 8, 9, 10, or higher. That is, the receiver 104 can include any numberof photodiodes 106 as described herein.

FIG. 2 illustrates an embodiment of a photodiode 200 configured as anavalanche photodiode (APD). The photodiode 200 can include a mesa region201. The APD 200 can include an avalanche region 202 that is close to anoptical entrance surface 204 of a window layer 206. The window layer 206and at least a portion of the avalanche region 202 can be included inthe mesa region 201. The window layer 206 can include an anode 203,which can be an annular anode as shown. The annular anode 203 can definean optical aperture 205 by an aperture 207 of the annular anode 203. Theoptical aperture 205 extends longitudinally with respect to alongitudinal axis (e.g., parallel to the longitudinal axis) of theaperture 207 of the annular anode 203. The sides of the optical aperture205 can be defined by the sides of the annular anode 203 that define theaperture 207.

The window layer 206 can have a thickness of about 0.5 microns to about1.5 microns, from about 0.7 microns to about 1.2 microns, or from about0.85 microns to about 1.0 micron. The width of the window layer 206 canbe about 36 microns to about 116 microns, from about 46 microns to about96 microns, or from about 56 microns to about 76 microns. Depending onthe wavelength of the APD 200, the window layer 206 thickness can affectthe responsivity of the APD 200. For 850 nm, the optimum window layerthickness can be about 0.852 microns. For any given APD design it can bedesirable to keep the area of the window layer 206 after mesa etch to aminimum to realize the minimum junction capacitance for the active areaof that particular design. These parameters can be used in anyembodiment of the avalanche photodiode described herein.

The annular anode 203 metal contact can have a thickness of about 0.5microns to about 1.5 microns, from about 0.6 microns to about 1.4microns, or from about 0.7 microns to about 1.2 microns. The outerdimension of the annular anode 203 can be about 30 microns to about 110microns, from about 40 microns to about 90 microns, or from about 50microns to about 70 microns. The dimension of the aperture 207 of theannular anode 203 metal contact can be about 20 microns to about 100microns, from about 30 microns to about 80 microns, or from about 40microns to about 60 microns. The anode metal thickness can be modulated.In one aspect, the same anode metal can be on both VCSELs andphotodiodes. A wide variety of anode metal thickness values can be usedwith equal performance. In one aspect, a minimum anode contact openingwidth of 3.0 microns can be beneficial. In one aspect, the anode metalcan be about 1.0 micron larger than the anode contact opening (e.g., theaperture 207) to allow for alignment errors. This means that the minimumwidth of the anode metal can be 5.0 microns. The outer dimension of theanode metal is the aperture 207 dimension plus an additional 5.0microns. That is, the distance from outer edge to inner edge is about 5microns. In one aspect, the annular anode 203 is annular; however, theannular anode 203 can be a partial aperture, “C” shaped, or arc, andthereby the annular anode 203 does not have to go all the way around theactive area of the optical aperture 205, which helps to achieve aminimum junction capacitance. These parameters can be used in anyembodiment of the avalanche photodiode described herein.

The avalanche region 202 can be a GaAs avalanche region 202 and can beclose to the optical entrance at the optical entrance surface 204because of low hole mobility of GaAs materials to preserve the opticalbandwidth (BW) of the APD 200.

In an example of receiving about 850 nm light to the optical entrancesurface 204, the window layer 206 can be a P⁺ AlGaAs material. Also, thewindow layer 206 can be about 0.852 microns thick, or +/−1%, 2%, 5%,10%, 15%, or 20% thereof. These parameters can be used in any embodimentof the avalanche photodiode described herein.

In one example, the window layer 206 can include an Al_(0.15)Ga_(0.85)AsP-type window layer. However, the mole fractions can vary betweenAl_(0.12)Ga₀₈₈As to Al_(0.4)Ga₀₆, or between Al_(0.13)Ga_(0.87)As toAl_(0.35)Ga_(0.65), or between Al_(0.15)Ga_(0.85)As to Al_(0.3)Ga_(0.7).In one aspect, the window layer 206 can be transparent or substantiallytransparent to the wavelength of the light for a particular APD 200. Iftoo much Al is used in the AlGaAs of the window layer 206, the holemobility can be reduced and the series resistance can increase. For eachwavelength there is an optimum window layer composition, which can bedesigned as desired. These parameters can be used in any embodiment ofthe avalanche photodiode described herein.

The avalanche region 202 is adjacent or contacting a surface of thewindow layer 206 that is opposite of the optical entrance surface 204.The optical entrance surface 204 receives light 208 that passes throughthe window layer 206 and into the avalanche region 202. The avalancheregion 202 includes an N-type layer 210 below and optionally directlyadjacent with and optionally in contact with the window layer 206opposite of the optical entrance surface 204. The N-type layer 210 canbe optionally patterned by use of: a) interrupted growth and mesa etch;b) proton implant; or c) Be implant. The N-type layer 210 can beprepared with moderately heavy doping (˜1.3×10¹⁷ cm⁻³) with silicon orother donor impurity.

Also, the N-type layer 210 can be dimensioned such that it is onlypresent in the optical aperture 205. That is, the sides of the N-typelayer 210 can be aligned with the sides of the optical aperture 205, andthereby be aligned with the sides of the annular anode 203 that definethe aperture 207. The N-type layer 210 can be only present in theoptical aperture 205 so that it does not extend laterally past the sidesof the optical aperture (e.g., the N-type layer 210 is not below thebody of the annular anode 203, but only below the aperture 207 of theannular anode 203). The N-type layer 210 can be prepared from GaAs thatis N-doped. The N-type layer 210 can have a thickness of about 0.2microns to about 0.4 microns, from about 0.3 microns to about 0.38microns, or from about 0.35 microns to about 0.36 microns. The width ofthe N-type layer 210 after patterning can be about 20 microns to about100 microns, from about 30 microns to about 80 microns, or from about 40microns to about 60 microns. However, it may not be patterned and it mayextend across the entire mesa. The N-type 210 layer can be consideredthe active layer. These parameters can be used in any embodiment of theavalanche photodiode described herein.

When an EPI is grown, the N-type layer 210 can be deposited over theentire surface of the wafer (e.g., over an N⁻ absorption layer 216). Forperformance in the APD 200, it can be desirable to have the N-type layer210 only present in the active area. With the interrupted growthapproach, the EPI process is stopped after the N-type layer 210 isdeposited. The wafer is removed from the EPI reactor and patterned withphotoresist to define the active area (e.g., the optical aperture 205)and the excess N-type material of the N-type layer 210 is etched away,and the etch is stopped on the N⁻ absorption layer 216 below the N-typelayer 210. Then the wafer is put back in the EPI reactor, and aconformal P-type window layer 206 is deposited on the wafer onto theexposed portion of the N⁻ absorption layer 216 and the N-type layer 210.The resulting mesa defines the location of the active area (e.g., theoptical aperture 205), and allows subsequent processes to be aligned tothat active area structure. When the proton or Be implant is used, allthe EPI layers including the P-type window layer 206 are grown in theEPI reactor over the entire surface of the wafer. Then the wafer ispatterned with photoresist to protect the active area and the otherparts of the wafer are implanted with protons or Be atoms at anappropriate energy to penetrate through the P-type window layer 206, andcompensate the donor concentration in the N-type layer 210. With theseapproaches, the result is to increase the breakdown voltage of a higherbreakdown voltage spacer region 212 around the active area so that theavalanche multiplication region is confined to the active area (e.g.,the optical aperture 205). The thickness and doping level in the N-typelayer 210 can be modulated and used to determine the breakdown voltageof the APD 200. In one aspect, an operating voltage of ˜15 V can bebeneficial; however, the value of the operating voltage can be anywherebetween 12 V and 25 V for a useful APD 200.

The avalanche region 202 can optionally include the higher breakdownvoltage spacer region 212 laterally from the N-type layer 210 to a mesasurface 214 of the mesa region 201. The higher breakdown voltage spacerregion 212 can be a P⁺—N⁻—N⁺ material with a higher breakdown voltagethan the N-type layer 210. The higher breakdown voltage is accomplishedby compensating the N-type layer 210 with either proton implant or Beimplant. The higher breakdown voltage spacer region 212 can have athickness of about the same as the N-type layer 210 or 0.2 microns toabout 0.4 microns, from about 0.3 microns to about 0.38 microns, or fromabout 0.35 microns to about 0.36 microns. The width of the higherbreakdown voltage spacer region 212 can be about 7.0 microns to about9.0 microns, from about 7.5 microns to about 8.5 microns, or from about7.9 microns to about 8.1 microns. The width of the higher breakdownvoltage spacer region 212 can be determined by the diameter of theactive area, and the buildup of dimensions required by the anode contactand the sloped mesa etch. In one aspect, the junction area can be keptto a minimum to minimize junction capacitance while keeping thedepletion region at the operating voltage from reaching the mesa surface214. In one example, avalanche happens at about 740 kV/cm. For a 15 Vbreakdown a 0.2-micron depletion width may be suitable. However, thehigher breakdown voltage spacer region 212 can be omitted, and theN-type layer 210 can extend across the entire mesa from surface tosurface.

The avalanche region 202 can include the N⁻ absorption layer 216 that isbelow and optionally directly adjacent with and optionally in contactwith the N-type layer 210. When the higher breakdown voltage spacerregion 212 is included, the N⁻ absorption layer 216 is below andoptionally directly adjacent with and optionally in contact with thehigher breakdown voltage spacer region 212. As such, the N⁻ absorptionlayer 216 can extend across the mesa region 201 to the mesa surface 214,and thereby be under both the N-type layer 210 and the higher breakdownvoltage spacer region 212. The N⁻ absorption layer 216 can be GaAs. TheN⁻ absorption layer 216 can be distinguished from the N-type layer 210by the lower donor concentration of <1.174×10¹⁵ cm⁻³. Input photons areabsorbed in both the N-type layer 210 and the N⁻ absorption layer 216.The total thickness of the N-type layer 210 and the N⁻ absorption layer216 determines the responsivity, avalanche voltage, and the bandwidth ofthe APD 200. For a total thickness of the N-type layer 210 plus the N⁻absorption layer 216 of 3.0 microns, the bandwidth can be about 10 GHzwith responsivity of 0.64 mA/mW; for a total thickness of 2.1 micronsthe bandwidth can be about 14 GHz with responsivity of 0.58 mA/mW; for atotal thickness of 1.2 microns the bandwidth can be about 28 GHz withresponsivity of 0.4 mA/mW. All of these APD configurations can have anavalanche voltage of 12 V to 25 V by adjusting the thickness and dopingof the N-type layer 210. These parameters can be used in any embodimentof the avalanche photodiode described herein.

An N⁺ barrier layer 218 is below and optionally directly adjacent withand optionally in contact with the N⁻ absorption layer 216. As such, theN⁺ barrier layer 218 can extend across the mesa region 201 to the mesasurface 214, and thereby be under the N⁻ absorption layer 216. The N⁺barrier layer 218 can be N⁺ AlGaAs. The N⁺ barrier layer 218 can beN-doped. The N⁺ barrier layer 218 can have AlGaAs asAl_(0.15)Ga_(0.85)As. However, the mole fractions of the N⁺ barrierlayer 218 can vary between Al_(0.12)Ga_(0.88)As to Al_(0.13)Ga_(0.87),or between Al_(0.35)Ga_(0.65)As to Al_(0.15)Ga_(0.85), or betweenAl_(0.15)Ga_(0.85)As to Al_(0.3)Ga_(0.7)As. The N⁺ barrier layer 218 canhave a thickness of about 0.25 microns to about 0.5 microns, from about0.3 microns to about 0.4 microns, or from about 0.32 microns to about0.35 microns, or about 0.32 microns. The width of the N⁺ barrier layer218 can be about 40 microns to about 120 microns, from about 50 micronsto about 100 microns, or from about 60 microns to about 80 microns. TheN⁺ barrier layer 218 can be transparent to the design wavelength of theAPD 200. Typically, the N⁺ barrier layer 218 can have the same Al/Garatio as the window layer 206. The N⁺ barrier layer 218 can serve toblock any holes optically generated in an N⁺ conduction layer 220 fromcontributing to the APD response. Holes generated in the N⁺ conductionlayer 220 may limit the bandwidth of the APD and should be excluded.These parameters can be used in any embodiment of the avalanchephotodiode described herein.

The N⁺ conduction layer 220 is below and optionally directly adjacentwith and optionally in contact with the N⁺ barrier layer 218. The N⁺conduction layer 220 can have a top portion that is part of the mesaregion 201 and a bottom portion that is not part of the mesa region 201(e.g., extend onto a solder region 226). The mesa etch normally extendsabout 1.0 micron into the N⁺ conduction layer 220. As such, the N⁺conduction layer 220 can have a top portion that extends across the mesaregion 201 to the mesa surface 214, and thereby be under the N⁺ barrierlayer 218, and a bottom portion that extends laterally outward past themesa surface 214 of the mesa region 201. The N⁺ conduction layer 220 canbe N⁺ GaAs. The N⁺ conduction layer 220 can be N-doped. The N⁺conduction layer 220 can have a top portion with a thickness of about0.7 microns to about 1.2 microns, from about 0.85 microns to about 1.15microns, or from about 0.95 microns to about 1.05 microns, or about 1.0microns. The width of the top portion of the N⁺ conduction layer 220 canbe about 40 microns to about 120 microns, from about 50 microns to about100 microns, or from about 60 microns to about 80 microns. The N⁺conduction layer 220 can have a bottom portion with a thickness of about1.8 microns to about 2.2 microns, from about 1.9 microns to about 2.1microns, or from about 1.95 microns to about 2.05 microns, or about 2microns. The width of the bottom portion of the N⁺ conduction layer 220extends to the edge of the chip after singulation by sawing. For asingle APD die, the size is about square with dimensions of about 200microns to about 250 microns, from about 210 microns to about 240microns, or from about 212 microns to about 215 microns. For an array ofAPDs the length of the array will depend on the number of elements inthe array; the width will always be the same. These parameters can beused in any embodiment of the avalanche photodiode described herein.

A substrate 222 is below and optionally directly adjacent with andoptionally in contact with the N⁺ conduction layer 220. The substrate222 can be an N-type GaAs substrate or a semi-insulating GaAs (S—IGaAs). An N-type GaAs substrate 222 can be N-doped. An S—I GaAssubstrate 222 can be doped with a deep level impurity like Cr which pinsthe Fermi level near mid bandgap and results in a resistivity of˜1.0×10⁷Ω-cm. These parameters can be used in any embodiment of theavalanche photodiode described herein.

A cathode metal Ohmic contact 224, which can be an annular cathode orsemicircle or arc or “C” shaped, can be located on the N⁺ conductionlayer 220. The cathode metal Ohmic contact 224 can be on a surface ofthe N⁺ conduction layer 220 that is not part of the mesa region 201. Assuch, the N⁺ conduction layer 220 can have a top portion that is part ofthe mesa region 201 and a bottom portion that is not part of the mesaregion 201, and such bottom portion can extend laterally from the mesasurface 214 and be referred to as the solder region 226. The cathodemetal Ohmic contact 224 is deposited, patterned, and alloyed to form thecathode Ohmic contact. The cathode metal Ohmic contact 224 can have athickness of about 0.2 microns to about 0.6 microns, from about 0.25microns to about 0.5 microns, or from about 0.3 microns to about 0.33microns. In one aspect, a minimum width of the cathode contact openingcan be 6.0 microns and the cathode metal can be 1.5 microns larger thanthe contact opening to allow for alignment errors. This can result in aminimum anode metal width of 9.0 microns. The cathode metal Ohmiccontact 224 can be annular, but it does not extend all the way aroundthe mesa, and one example can be a typical N-contact arc. The cathodepad metal is deposited over the cathode metal Ohmic contact 224. Thecathode pad metal can be 1.0 micron larger than the cathode metal Ohmiccontact 224 to allow for alignment errors. The minimum width of thecathode pad metal is 11.0 microns. The cathode pad metal can be 10microns away from the layout dimension of the mesa to facilitatelift-off of the cathode pad metal. Thus, the inner and outer dimensionsof the cathode metal Ohmic contact 224 and cathode pad metal aredetermined by design rules dictated by the process and the dimension ofthe mesa. These parameters can be used in any embodiment of theavalanche photodiode described herein.

In one embodiment, the cathode pad metal can be identical with the anodepad and contact metal that is described herein. Before the cathode padmetal is deposited, there is a deep proton implant process that convertsmuch of the N⁺ conduction layer 220 to semi-insulating in the area underthe cathode and anode bond pads. The purpose of this proton implant isto minimize the anode pad capacitance. Both the anode bond pad andcathode bond pads are over dielectric on semi-insulating GaAs. In ahigh-speed photodiode, it is important to keep the total capacitance(junction+pad capacitance) at the lowest possible value for bestperformance. In one aspect, the deep proton implant can also be used toimplant a portion of the mesa that is outside the active area. This useof the proton implant reduces junction capacitance.

In one embodiment, a passivation dielectric material can be deposited onthe entire wafer as a passivation layer to provide protection fromhumidity when the APD 200 is not in a hermetic package. This passivationlayer is typically a combination of silicon dioxide and silicon nitride.The thickness of the passivation layer can be tailored to the wavelengthof the APD to give maximum responsivity. This is accomplished by havingthe total thickness of dielectric in the active region of the APD be anodd multiple of ¼ the design wavelength for that APD; typically thetotal thickness in the active area is ¾λ or 5/4λ.

In one embodiment, a Ti-etch can be included in the bond pad area. Toallow the passivation dielectric to bond to the pad metal, both theanode contact and pad metal and the cathode pad metal are terminatedwith a thin layer of Ti. This Ti layer is removed in the bonding area onboth the anode and cathode bond pads before the passivation dielectricis deposited. The Ti layer can be removed in the bonding area to allowgood electrical contact with the ball bond on the anode and cathode bondpads.

In one embodiment, the N-type layer 210 needs to be higher conductivitythan the higher breakdown voltage spacer region 212, which can beaccomplished by making the N-type layer 210 to have higher conductivityby putting in an implant into the N-type layer 210 that increases itsconductivity and leaving the higher breakdown voltage spacer region 212as the deposited material without implantation. Alternatively, as shownin FIG. 3, an N-type layer 310 may not be implanted, but an implantregion 312 is implanted so as to inhibit or stop conductivity (e.g.,with beryllium implantation) in the higher voltage breakdown spacerregion so that higher conductivity stays in the center of an APD 300 inthe area of the N-type layer 310. This can make the N-type layer 310more conductive than the implant region 312.

In one embodiment, the APD can be prepared so that the N-type layer 210is more conductive than the region that surrounds the N-type layer 210(e.g., the higher breakdown voltage spacer region 212).

In one example, an avalanche breakdown voltage for a P⁺N GaAs PNjunction of 15 V can have a peak E-field at the junction of ˜7.4×10⁵V/cm. This can be achieved with an N-type layer doped to 1.3×10¹⁷ cm⁻³.At the breakdown voltage of 15 V, the depletion layer thickness can be0.405 microns. For an 850-nm APD, a GaAs absorption region thickness of0.405 microns can result in a low optical responsivity (˜0.21 mA/mW) andhigh junction capacitance per unit area.

In one embodiment, the windows (e.g., P⁺ window layers) and blockinglayers (e.g., N⁺ barrier layers) of the APD can be transparent, such asbeing transparent to the intended wavelength of operation of the APD.While the window and blocking layers can be transparent to anywavelength of light, it can be advantageous for the window and blockinglayers to be transparent to the intended wavelength of operation and maybe non-transparent to other wavelengths that are significantly differentfrom the intended wavelength. Absorption in the window and blockinglayers may be limited to free carrier absorption due to high impurityconcentration, and should not be band-to-band absorption that generateshole-electron pairs. Such band-to-band absorption outside the activethickness can reduce the signal bandwidth of the APD, and should beavoided.

FIG. 3 illustrates another embodiment of a photodiode 300 configured asan avalanche photodiode 300. The photodiode 300 can include a mesaregion 301. The APD 300 can include an avalanche region 302 that isclose to an optical entrance surface 304 of a window layer 306. Thewindow layer 306 and at least a portion of the avalanche region 302 canbe included in the mesa region 301. The window layer 306 can include ananode 303, which can be an annular anode as shown. An annular anode 303can define an optical aperture 305 by an aperture 307 of the annularanode 303. The optical aperture 305 extends longitudinally with respectto a longitudinal axis (e.g., parallel to the longitudinal axis) of theaperture 307 of the annular anode 303. The sides of the optical aperture305 can be defined by the sides of the annular anode 303 that define theaperture 307. The dimensions and compositions and other parameters ofthe APD 300 can be substantially as described with respect to the APD200 of FIG. 2.

The avalanche region 302 can be a GaAs avalanche region 302 and can beclose to the optical entrance at the optical entrance surface 304because of low hole mobility of GaAs materials to preserve the opticalbandwidth (BW) of the APD 300.

In an example of receiving about 850-nm light to the optical entrancesurface 304, the window layer 306 can be a P⁺ AlGaAs material. Also, thewindow layer 306 can be about 0.852 microns thick, or +/−1%, 2%, 5%,10%, 15%, or 20% thereof.

In one example, the window layer 306 can include an Al_(0.15)Ga_(0.85)AsP-type window layer. However, the mole fractions can vary betweenAl_(0.12)Ga₀₈₈As to Al_(0.4)Ga₀₆, or between Al_(0.13)Ga_(0.87)As toAl_(0.35)Ga_(0.65), or between Al_(0.15)Ga_(0.85)As to Al_(0.3)Ga_(0.7).In one aspect, the window layer 306 can be transparent or substantiallytransparent to the wavelength of the light for a particular APD 300. Iftoo much Al is used in the AlGaAs of the window layer 306, the holemobility can be reduced and the series resistance can increase. For eachwavelength there is an optimum window layer composition, which can bedesigned as desired.

The avalanche region 302 is adjacent or contacting a surface of thewindow layer 306 that is opposite of the optical entrance surface 304.The optical entrance surface 304 receives light 308 that passes throughthe window layer 306 and into the avalanche region 302. The avalancheregion 302 includes the N-type layer 310 below and optionally directlyadjacent with and optionally in contact with the window layer 306opposite of the optical entrance surface 304. The N-type layer 310 canbe patterned and can be prepared with moderately heavy doping. Also, theN-type layer 310 can be dimensioned such that it is only present in theoptical aperture 305. That is, the sides of the N-type layer 310 can bealigned with the sides of the optical aperture 305, and thereby bealigned with the sides of the annular anode 303 that define the aperture307. The N-type layer 310 can be only present in the optical aperture305 so that it does not extend laterally past the sides of the opticalaperture (e.g., the N-type layer 310 is not below the body of theannular anode 303, but only below the aperture 307 of the annular anode303). The N-type layer 310 can be prepared from GaAs that is N-doped.The N-type layer 310 layer can be considered the active layer.

The avalanche region 302 includes the implant region 312 laterally fromthe N-type layer 310 to a mesa surface 314 of the mesa region 301. Theimplant can be a proton implant or beryllium implant, or other implantthat reduces conductivity of the implant region 312. The implant region312 can be configured as a single energy proton implant or berylliumimplant that is around the perimeter of the optical aperture 305, andthereby under the body of the annular anode 303. The presence of theimplant region 312 can convert the top region of the avalanche region302 to be semi-insulating.

The avalanche region 302 can include an N⁻ absorption layer 316 that isbelow and optionally directly adjacent with and optionally in contactwith the N-type layer 310 and the implant region 312. As such, the N⁻absorption layer 316 can extend across the mesa region 301 to the mesasurface 314, and thereby be under both the N-type layer 310 and theimplant region 312. The N⁻ absorption layer 316 can be GaAs. The N⁻absorption layer 316 can be semi-insulating from the implant region 312,which can be similar to a gain guide implant process used in protonvertical cavity surface-emitting lasers (VCSELs).

The doping profile in the N⁻ absorption layer 316 (e.g., N-type GaAsabsorption layer) can be designed to give an avalanche voltage of about15 V with a lateral depletion region that does not reach the mesasurface 314 of the mesa region 301.

An N⁺ barrier layer 318 is below and optionally directly adjacent withand optionally in contact with the N⁻ absorption layer 316. As such, theN⁺ barrier layer 318 can extend across the mesa region 301 to the mesasurface 314, and thereby be under the N⁻ absorption layer 316. The N⁺barrier layer 318 can be N⁺ AlGaAs. The N⁺ barrier layer 318 can beN-doped. The N⁺ barrier layer 318 can have N-doped AlGaAs asAl_(0.15)Ga_(0.85)As.

An N⁺ conduction layer 320 is below and optionally directly adjacentwith and optionally in contact with the N⁺ barrier layer 318. The N⁺conduction layer 320 can have a top portion that is part of the mesaregion 301 and a bottom portion that is not part of the mesa region 301such as being at or forming a shoulder 326 that extends laterally fromthe mesa. As such, the N⁺ conduction layer 320 can have a top portionthat extends across the mesa region 301 to the mesa surface 314, andthereby be under the N⁺ barrier layer 318, and a bottom portion thatextends laterally outward past the mesa surface 314 of the mesa region301 to the shoulder 326. The N⁺ conduction layer 320 can be N⁺ GaAs. TheN⁺ conduction layer 320 can be N-doped.

A substrate 322 is below and optionally directly adjacent with andoptionally in contact with the N⁺ conduction layer 320. The substrate322 can be an N-type GaAs substrate or a semi-insulating GaAs (S—IGaAs). An N-type GaAs substrate 322 can be N-doped.

A cathode 324, which can be an annular cathode, can be located on the N⁺conduction layer 320. The cathode 324 can be on a surface of the N⁺conduction layer 320 that is not part of the mesa region 301. As such,the N⁺ conduction layer 320 can have a top portion that is part of themesa region 301 and a bottom portion that is not part of the mesa region301, and such the bottom portion can extend laterally from the mesasurface 314 and be referred to as the shoulder 326 region.

In view of the embodiment of the avalanche photodiode in FIG. 3, the N⁺barrier layer 318 can be an Al_(0.15)Ga_(0.85)As layer and be locateddirectly below the N⁻ absorption layer 316 that is GaAs in order to keepoptically-generated carriers from the N⁺ conduction layer 320 of GaAsmaterial from contributing to the response of the photodiode. Excludingthese carriers gives the maximum optical bandwidth for the APD 300.

In one aspect, the implant region 312 can cause some shallow damage, andwhich damage can be overcome by doping compensation near the surface soas to achieve a good P-type Ohmic contact in the implant region 312.However, the implant region 312 can be excluded.

The configuration of the APD 300 can provide an avalanche gain processthat is optimized when the avalanche region 302 is confined to a narrowregion adjacent to a P-type window layer 306 by using nonuniform dopingof the N⁻ absorption layer 316 of GaAs material. The N-type doping inthe GaAs material of the N⁻ absorption layer 316 can be highest adjacentto or closest to the P-type window layer 306, and the N-type doping canbe comparatively lower through the remainder of the N⁻ absorption layer316. The energy and dose of the implant region 312 is selected tocompensate this heavier doped region of the N⁻ absorption layer 316 tobecome semi-insulating.

By making the aperture 307 of the annular anode 303 (e.g., metal anodematerial) and the aperture of a single energy proton implant region 312the same dimensions and longitudinally aligned, the avalanche gain canbe confined to the central region of optical input, which can beconsidered the optical aperture 305. The aperture of a proton implantregion 312 can be the N-type layer 310. Accordingly, the N-type layer310 can be the N-type doping region of the N⁻ absorption layer 316 thatis the highest, as described above. The annular region of the implantregion 312 can extend about 10 microns from the mesa surface 314 of thetop of the mesa region 301. This configuration can provide for thereverse bias depletion region not reaching the radial surface of themesa.

Accordingly, the GaAs material of the N⁻ absorption layer 316 can have atop middle portion (e.g., aperture) that is heavier doped so as to formthe N-type layer 310 and a top outer portion (e.g., annular) that isconfigured as the implant region 312.

In one embodiment, the N-type layer 310 can be grown across wafer, andthen converted to the implant region 312 by implantation of a componentthat reduces conductivity in the implant region 312 compared to theN-type layer 310. For example, the implant region 312 can be formed byproton or beryllium implantation therein. As such, the implant region312 includes a conductivity reducing implant.

In one embodiment as illustrated in FIG. 3A, the avalanche photodiode300 can have a modification where the N-type layer 310 extends all theway across the photodiode 300 as a continuous layer. Accordingly, theN-type layer 310 may not have a lateral implant region (e.g., a lateralimplant region 312 of FIG. 3). It has been found that having the N-typelayer 310 extending all the way across the mesa region 301 between themesa surface 314 provides good performance, even without implanting thelateral regions. As such, the N-type layer 310 can form a portion of themesa surface 314. Thus, the photodiode can be devoid of an implantregion laterally of the N-type layer 310. Other than dimension andarrangement, the material of the N-type layer 310 can be the same asdescribed in connection with FIG. 3. This can include the N-type layer310 being prepared from GaAs that is N-doped. The sloped edge of theN-type layer 310 can be about 60-65 degrees (e.g., +/−1%, 2%, 5%, 10%,15%, or 20% thereof), which angle can be used for the entire mesa. Theconfiguration of the N-type layer 310 has been established to besufficient to lower the field of the surface so that there is not anyedge breakdown.

Now, manufacturing can be simplified because the N-type layer 310 doesnot have the lateral region etched or proton implanted. Accordingly, theepitaxial growth of the N-type layer of Step 1 of FIG. 5 can be used,and the Step 2 can be omitted.

FIG. 4 illustrates another embodiment of a photodiode 400 configured asan avalanche photodiode 400. The avalanche photodiode 400 can besubstantially as in FIG. 2 or 3; however, the active region can be aP-type active region, which can be prepared substantially similarly asdescribed herein with the exception that the processing inverts thelocation of any beryllium implant. As such, the dimensions andconfigurations of the P-type active region can be the same as thedimensions and configurations of the N-type active region describedherein. In the photodiode 400, the beryllium implant is now within theactive region. The photodiode 400 can include a mesa region 401. An APD400 can include an avalanche region 402 that is close to an opticalentrance surface 404 of a window layer 406. The window layer 406 and atleast a portion of the avalanche region 402 can be included in the mesaregion 401. The window layer 406 can include an anode 403, which can bean annular anode as shown or “C” shaped anode, or any arced shape. Theanode 403 can define an optical aperture 405 by an aperture 407 of theanode 403. The optical aperture extends longitudinally with respect to alongitudinal axis (e.g., parallel to the longitudinal axis) of theaperture 407 of the anode 403. The sides of the optical aperture 405 canbe defined by the sides of the anode 403 that define the aperture 407.The dimensions and compositions and other parameters of the APD 400 canbe substantially as described with respect to the APD 200 of FIG. 2.

The avalanche region 402 can be a GaAs avalanche region 402 and can beclose to the optical entrance at the optical entrance surface 404because of low hole mobility of GaAs materials to preserve the opticalbandwidth (BW) of the APD 400.

In an example of receiving about 850-nm light to the optical entrancesurface 404, the window layer 406 can be a P⁺ AlGaAs material. Also, thewindow layer 406 can be about 0.852 microns thick, or +/−1%, 2%, 5%,10%, 15%, or 20% thereof.

In one example, the window layer 406 can include an Al_(0.15)Ga_(0.85)AsP-type window layer. However, the mole fractions can vary betweenAl_(0.12)Ga₀₈₈As to Al_(0.4)Ga₀₆, or between Al_(0.13)Ga_(0.87)As toAl_(0.35)Ga_(0.65), or between Al_(0.15)Ga_(0.85)As to Al_(0.3)Ga_(0.7).In one aspect, the window layer 406 can be transparent or substantiallytransparent to the wavelength of the light for a particular APD 400. Iftoo much Al is used in the AlGaAs of the window layer 406, the holemobility can be reduced and the series resistance can increase. For eachwavelength there is an optimum window layer composition, which can bedesigned as desired.

The avalanche region 402 is adjacent or contacting a surface of thewindow layer 406 that is opposite of the optical entrance surface 404.The optical entrance surface 404 receives light 408 that passes throughthe window layer 406 and into the avalanche region 402.

The avalanche region 402 can include a P⁻ absorption layer 416 below andoptionally directly adjacent with and optionally in contact with thewindow layer 406 opposite of the optical entrance surface 404. The P⁻absorption layer 416 can have a center region under the optical aperture405 that is not implanted, and can include an implant region 412 that isimplanted, such as by proton implanting.

The avalanche region 402 includes a P-type layer 410 within or below theP⁻ absorption layer 416. The P-type layer 410 can be patterned and canbe prepared with moderately heavy doping. Also, the P-type layer 410 canbe dimensioned such that it is only present in the optical aperture 405.That is, the sides of the P-type layer 410 can be aligned with the sidesof the optical aperture 405, and thereby be aligned with the sides ofthe anode 403 that define the aperture 407. The P-type layer 410 can beonly present in the optical aperture 405 so that it does not extendlaterally past the sides of the optical aperture (e.g., the P-type layer410 is not below the body of the anode 403, but only below the aperture407 of the anode 403). The P-type layer 410 can be prepared from GaAsthat is P-doped. The P-type layer 410 can have more P doping compared tothe P⁻ absorption layer 416. As such, the P⁻ absorption layer 416 can beP⁻ and the P-type layer 410 can be P⁺. The P-type layer 410 can beconsidered the active layer.

Optionally, the P-type layer 410 can include a beryllium implant 411.

In another option, the P-type layer 410 can extend across the mesaregion 401 so that sides of the P-type layer 410 form a portion of mesasides 414. Here, the implant region 412 can be omitted. As such, theP-type layer 410 can be configured as the N-type layer of FIG. 3A.

The avalanche region 402 includes the implant region 412 laterally fromthe P⁻ absorption layer 416 and laterally from the P-type layer 410 to amesa surface 414 of the mesa region 401. The implant can be a protonimplant or other implant that reduces conductivity of the implant region412. The implant region 412 can be configured as a single energy protonimplant that is around the perimeter of the optical aperture 405, andthereby under the body of the anode 403. The presence of the implantregion 412 can convert the top region of the avalanche region 402 to besemi-insulating.

The doping profile in the P⁻ absorption layer 416 (e.g., P-type GaAsabsorption layer) can be designed to give an avalanche voltage of about15 V with a lateral depletion region that does not reach the mesasurface 414 of the mesa region 401.

An N⁺ barrier layer 418 is below and optionally directly adjacent withand optionally in contact with the P⁻ absorption layer 416 and theP-type layer 410. As such, the N⁺ barrier layer 418 can extend acrossthe mesa region 401 to the mesa surface 414, and thereby be under the P⁻absorption layer 416 and the P-type layer 410. The N⁺ barrier layer 418can be N⁺ AlGaAs. The N⁺ barrier layer 418 can be N-doped. The N⁺barrier layer 418 can have N-doped AlGaAs as Al_(0.15)Ga_(0.85)As.

An N⁺ conduction layer 420 is below and optionally directly adjacentwith and optionally in contact with the N⁺ barrier layer 418. The N⁺conduction layer 420 can have a top portion that is part of the mesaregion 401 and a bottom portion that is not part of the mesa region 401.As such, the N⁺ conduction layer 420 can have a top portion that extendsacross the mesa region 401 to the mesa surface 414, and thereby be underthe N⁺ barrier layer 418, and a bottom portion that extends laterallyoutward past the mesa surface 414 of the mesa region 401. The N⁺conduction layer 420 can be N⁺ GaAs. The N⁺ conduction layer 420 can beN-doped.

A substrate 422 is below and optionally directly adjacent with andoptionally in contact with the N⁺ conduction layer 420. The substrate422 can be an N-type GaAs substrate or a semi-insulating GaAs (S—IGaAs). An N-type GaAs substrate 422 can be N-doped.

A cathode 424, which can be an annular cathode, can be located on the N⁺conduction layer 420. The cathode 424 can be on a surface of the N⁺conduction layer 420 that is not part of the mesa region 401 such thatthe cathode 424 is on a shoulder 426. As such, the N⁺ conduction layer420 can have a top portion that is part of the mesa region 401 and abottom portion that is not part of the mesa region 401, and such bottomportion can extend laterally from the mesa surface 414 and be referredto as the shoulder 426 region.

In one embodiment, proton implants are used to convert conductingmaterial to nonconducting material. This is used to guide current toparticular parts of the structure. The perimeter regions between thecentral active regions and mesa surface can include proton implants asdescribed herein.

In one embodiment, beryllium implants can be used in the perimeterregions to reduce current in the perimeter regions. Beryllium implantsare P-type, and can be used to compensate N-type material (e.g.,decrease conduction, such as in perimeter regions) or enhance P-typematerial (e.g., more conduction; see FIG. 4). In the structure of thepresent invention of FIG. 3, beryllium implants can be used to reducethe conductivity of the N-type perimeter area and create a higherbreakdown region to encourage avalanche detection in the center of thephotodiode.

FIG. 4A illustrates another embodiment of a photodiode 400 configured asan avalanche photo diode 400 that is similar to FIG. 4; however, theactive region can be a P-type active region, which can be preparedsubstantially similarly as described herein with the exception that theP-type layer 410 extends all the way across the mesa. As such, the P⁻absorption layer 416 and P-type layer 410 both extend across the mesa,and the P⁻ absorption layer 416 does not have a portion that is lateralof the P-type layer 410. Also, the P⁻ absorption layer 416 can be dopedwith about 1×10¹⁴ (e.g., 1e¹⁴); however the doping can range anddescribed herein or known in the art. The P-type layer 410 can be dopedwith about 1×10¹⁷ (e.g., 1e¹⁷) to about 2×10¹⁷ (e.g., 2e¹⁷); however thedoping can range and described herein or known in the art. Additionally,the window layer 406 can be a P⁺⁺ GaAlAs. This embodiment may optionallybe devoid of lateral implanting or include lateral implanting asdescribed herein. This embodiment may also optionally include theberyllium implant or be devoid thereof. The manufacture can be asdescribed herein; however, the P-type layer 410 is not etched.

FIG. 5 illustrates a process 500 for preparing an avalanche photodiodein accordance with the present invention. In one embodiment, the process500 includes Step 1, Step 2, and Step 3. However, Step 2 can be omittedin some instances, such as when the active region extends all the wayacross the mesa so as to not be etched.

Step 1 includes a process for growing a base epitaxial structure (EPI),which EPI includes: S—I GaAs substrate, an N⁺ GaAs N⁺ conduction layeron the S—I GaAs substrate, an N⁺ AlGaAs N⁺ barrier layer on the N⁺ GaAsN⁺ conduction layer, a N⁻ absorption layer on the N⁺ AlGaAs N⁺ barrierlayer, and an N⁺ GaAs N-type layer on the N⁻ absorption layer. Step 2includes a process for processing the EPI to leave a disc of N⁺material, which is the disc being the N⁺ GaAs N-type layer. Thedisc-shape material on the EPI can be considered a patterned EPI. Step 3includes a process for growing a P-type AlGaAs P-type region on top ofthe patterned EPI. As shown, a layer of substantially the same thicknessof a P-type AlGaAs material is grown as the P-type region over the N⁺GaAs N-type layer disc and over the N⁻ absorption layer. Since the dischas a thickness, the P-type region includes a stepped or raised portionover the disc compared to over the N⁻ absorption layer. Thus, the disccauses a plateau being formed in the P-type region because the samethickness of P-type AlGaAs is over the N⁻ absorption layer and disc. Theheight of the plateau can be the thickness of the disc. However,selective processing may result in there being no plateau of P-typeAlGaAs, as the P-type region may have a smooth or continuous surface orthe plateau as illustrated.

FIG. 6 illustrates a 1×4 APD array 600 having four different APDs 602 ina row. However, a wafer may have any reasonable number of APDs 602 on awafer. Individual APDs 602 can be configured as described herein, andcan include the features described in connection to FIG. 7. The APDarray 600 can be grown by the following process of Step 1 through Step24. However, some of the steps, such as etching the active region, canbe omitted.

FIG. 7 illustrates an embodiment of an APD chip 700 that includes theconfiguration of FIG. 3, such as a semi-insulating GaAs substrate 322,then the N⁺ GaAs conduction layer 320, then an N⁺ Al_(0.3)Ga_(0.7)Asbarrier layer 318, then an N⁻ GaAs absorption layer 316, then an N GaAsN-type layer 310, then the P⁺ GaAs/Al_(0.3)Ga_(0.7)As ramp (an interfaceregion 309 between 310 and 306 and between 312 and 306, see FIG. 3),then a P⁺ Al_(0.3)Ga_(0.7)As window layer 306, then a P⁺ GaAs cap layer311 (surface layer of the window layer 306, see FIG. 3).

Step 1 can include growing an APD EPI 710 on the semi-insulating GaAssubstrate 322, which can include: growing an N⁺ GaAs conduction layer320 to a thickness of about 3000 nm; thereon growing the N⁺Al_(0.3)Ga_(0.7)As barrier layer 318 (e.g., lower cladding layer about220 nm thick with GaAs/AlGaAs ramps (e.g., about 50 nm thick each) aboveand below so as to be about 320 nm thick, wherein the lower ramp isgrown, then the barrier layer is grown, and then the upper ramp is grownto provide the thickness of the N⁺ barrier layer 318); thereon growingthe N⁻ GaAs absorption layer 316 that is doped at about <1.0×10¹⁸ cm⁻³that is about 1745 nm thick; thereon growing the N GaAs N-type layer 310that is doped at about 1.3×10¹⁷ cm⁻³ that is about 355 nm thick; thereongrowing a P⁺ GaAs/Al_(0.3)Ga_(0.7)As ramp 309 that is doped at about1.0×10¹⁷-1.3×10¹⁹ cm⁻³ that is about 40 nm thick; and thereon growingthe P⁺ Al_(0.3)Ga_(0.7)As window layer 306 (e.g., upper cladding layer)doped at about 1.3×10¹⁹ cm⁻³ that is about 852 nm thick; and thereongrowing the P⁺ GaAs cap layer 311 that is doped at about 6.0×10¹⁹ cm⁻³that is about 40 nm thick. It should be recognized that the parametersof this step and the following steps can be modulated and adjusted forimprovement or optimization.

Table 1 shows parameters for an example of the APD EPI 710 obtainablefrom Step 1. Additional notes can include: the substrate beingsemi-insulating; the spacer layer being grown before a pause forreducing temperature, and can be high temperature NID; the absorptionlayer can be N-type with mobility >5000 cm²/Vs; and the N-type layer canbe NID. The example from Table 1 can be an 850-nm APD EPI, which can bein accordance with FIG. 12. It may also have an E-field versus thicknessplot as shown in FIG. 13.

Step 2 can include depositing an oxide layer 830 that is about 1450 A to2900 A thick onto the P⁺ GaAs cap layer 311. FIG. 8 shows an ADP EPI 810of a chip 800 on the substrate 322 having the oxide layer 830.

Step 3, which may be optional, can include etching the oxide layer 830to define a chip ID 712.

Step 4 can include applying a photoresist pattern onto the oxide layer830 to protect a central portion (e.g., a no Be or proton implant area718) of a mesa area 708 from Be or proton implantation.

Step 5 can include performing Be or proton implantation to the EPI inperimeter areas that are exposed (e.g., perimeter around centralportion) to form the implant region 312 (e.g., perimeter spacer region),where the photoresist covers and protects the central portion of theEPI, the Be or proton implantation being sufficient to lower net donorconcentration in the N GaAs N-type layer, which confines the avalanchebreakdown to the photoresist central protected area (e.g., centralportion) and excludes the avalanche breakdown from the perimeter areasaround the central portion. The central portion can be the active areaor optical active area. This step can be omitted.

Step 6 can include etching the EPI to form the mesa region 301, theetching being to a depth of about 1.0 microns into the N⁺ GaAsconduction layer 320, wherein the mesa slope of the mesa surface 314 isabout 60-70° or about +/−20%, 15%, 10%, 5%, 2.5%, or 1% thereof. Duringor after etching, the photoresist can be removed, or it can be retainedin the EPI.

Step 7 can include depositing a 145-nm silicon nitride dielectric overthe wafer, which can be a passivation layer 910. The silicon nitridedielectric material can be deposited by any process known in the art.FIG. 9 shows an embodiment with the mesa region 301 having thepassivation layer 910.

Step 8 can include applying a second photoresist pattern to thepassivation layer in order to protect a no proton implant area 706, theno proton implant area 706 being defined by the shape of the secondphotoresist pattern. The no proton implant area 706 can include thecentral portion of the mesa, and the no proton implant area 706 canexclude the perimeter areas around the central portion of the mesa. Theno proton implant area 706 can be shaped as defined in FIG. 7.

Step 9 can include performing multi-energy proton implantation of theEPI to convert the N⁺ GaAs conduction layer to semi-insulating layer.The N⁺ GaAs conduction layer is described in the FIGS. 2-4; however, itmay be referred to as the N⁺ GaAs implanted semi-insulating layer aftersuch proton implantation. However, N⁺ GaAs conduction layer may alsorefer to the semi-insulating layer when referring to the EPI or APDafter such proton implantation has been performed.

Step 10 can include applying a photoresist pattern to the no Be orproton implant area 718 so as to protect a central portion of mesa fromBe or proton implantation. Step 10A can include Be or protonimplantation.

Step 11 can include applying a third photoresist to the EPI with exposedareas that define an anode contact area 1010 and a cathode contact area1020. The anode contact area 1010 and the cathode contact area 1020 canbe annular or can be full or partial circles, and may be “C” shaped. Theanode contact area 1010 and the cathode contact area 1020 can be asillustrated and described herein.

Step 12 can include etching a 145-nm silicon nitride passivation layer910 to open anode and cathode contact areas. The etch may or may notalso be through a deposited oxide layer 830 that can be 1450 A to 2900A. As such the deposited oxide layer 830 may be exposed by the etching,or it may be etched to expose the P⁺ GaAs cap layer 311 and etch into aportion of the N⁺ conduction layer 320. During or after etching, thephotoresist can be removed, or it can be retained in the EPI. FIG. 10illustrates an etched anode contact area 1010 and the cathode contactarea 1020 in the passivation layer 910.

Step 13 can include applying a fourth photoresist to define a cathodecontact metal area 1020. The cathode contact metal area 1020 is definednow while an anode contact metal area 1010 is defined later as they maybe two different metal material compositions as is known in the art.

Step 14 can include deposition and lift-off of cathode contact metalinto the cathode contact metal area 1020 to form the cathode 324. Anyprocess can be used to form the cathode 324 as is known in the art. FIG.11 shows the cathode 324 in the cathode contact area 1020.

Step 15 can include forming an alloy cathode contact metal in thecathode contact metal area 1020. Any alloying process can be used toform the cathode alloy composition as is known in the art.

Step 16 can include applying a fifth photoresist to define an anode bondpad area 714 and the anode contact metal area 1010. The cathode areasare covered as the anode components need to be formed.

Step 17 can include deposition and lift-off of an anode bond pad 702 inthe anode bond pad area 714 and anode contact metal in the anode contactmetal area 1010. Any process can be used to form the anode contact andthe anode bond pad 702 as is known in the art. FIG. 11 shows the annularanode 303 in the cathode contact area 1020.

Step 18 can include applying a sixth photoresist to remove Ti in theanode bond pad area 714 and/or remove Ti from the anode contact metalarea 1010. This process can be done on an anode remove Ti andpassivation area 730.

Step 19 can include apply a seventh photoresist to define a cathode padmetal 722 area.

Step 20 can include deposition and lift-off of cathode bond pad metal inthe cathode bond pad areas to form cathode bond pads 704.

Step 21 can include applying an eight photoresist to remove Ti incathode bond pad areas and/or remove Ti from the cathode contact metalarea 1020. This process can be done on a cathode remove Ti andpassivation area 728.

Step 22 can include depositing a passivation layer 750 of siliconnitride/silicon dioxide dielectric material. The passivation layer 750can be over any portion or a whole chip 700.

Step 23 can include applying a ninth photoresist to define the anodebond pad 702 and define the cathode bond pads 704.

Step 24 can include removing the passivation layer 750 of dielectricmaterial in the anode bond pad 702 and in the cathode bond pad 704. Thiscan be by etching.

As such, the APD array 600 can include the layers and features obtainedby the manufacture process. The process can be by MBE or MOCVD.

The processing described herein may be modulated in accordance with theembodiments of the APDs described herein. The processing can beperformed in order to obtain an active region aligned with the N-typelayer (e.g., 210, 310, and 410), which can be a disc shape, where thereis an anti-reflection coating on the optical entrance. The finalthickness of any dielectric is going to be prepared in accordance withthe wavelength of light to be received by the APD. In one aspect, thewavelength can be 850 nanometers and the thickness can be determined sothat it is an anti-reflection coating that absorbs protons. Thethickness can be a multiple of a ¼ wavelength, such as ½, ¾, etc. of thewavelength. In one aspect, the dielectric can be a one-quarter lambdalayer in the active region and an environmental-protection layer (e.g.,passivation layer) can cover everything except the active region.Alternatively, the top environmental-protection layer can be applied tocover everything on the chip, including the active region, but its totalthickness over the active region is controlled to be an odd integermultiple of the quarter wavelength.

In one embodiment, an avalanche photodiode can include: an avalancheregion having one or more layers prepared from GaAs; an N⁻ absorptionlayer extending across the avalanche region; a N-type layer above acenter portion of the N⁻ absorption layer or extending across a mesaregion over the N⁻ absorption layer; and optionally a lower conductivitylayer laterally from the N-type layer to a surface of the avalancheregion and above a perimeter portion of the N⁻ absorption layer, thelower conductivity layer having lower conductivity compared to theN-type layer. In one aspect, the lower conductivity layer can have aconductivity-reducing implant compared to conductivity of the N-typelayer. In one aspect, the implant is a proton implant or a berylliumimplant. In one aspect, the avalanche photodiode can include a windowlayer above the N-type layer and lower conductivity layer, and an anodecontact above the window layer. In one aspect, the avalanche photodiodecan include an N⁺ barrier layer below the N⁻ absorption layer, an N⁺conduction layer below the N⁺ barrier layer, and a substrate below theN⁺ conduction layer, and a cathode contact coupled with the N⁺conduction layer. In one aspect, the avalanche region is included in amesa structure, the mesa extending into the N⁺ conduction layer so thata portion of the N⁺ conduction layer is in the mesa and a portion formsa shoulder from the mesa, with the cathode contact on the shoulder ofthe N⁺ conduction layer. In one aspect, the avalanche photodiode caninclude: an oxide layer above the window layer; a dielectric passivationlayer coated onto the window layer and over side surfaces of the mesa;an anode bond pad coupled with the anode contact; a cathode bond padcoupled with the cathode contact; and an environmental-protectionpassivation layer above the dielectric passivation layer, anode contact,and cathode contact, and the environmental-protection passivation layerhaving openings that expose the anode bond pad and cathode bond pad.

In one embodiment, an avalanche photodiode can include: an avalancheregion having one or more layers prepared from GaAs; an N⁻ absorptionlayer extending across the avalanche region; a N-type layer above acenter portion of the N⁻ absorption layer or extending over the entiretythereof; and optionally a lower conductivity layer laterally from theN-type layer to a surface of the avalanche region and above a perimeterportion of the N⁻ absorption layer, the lower conductivity layer havinglower conductivity compared to the N-type layer. The avalanchephotodiode can include a window layer above the N-type layer and abovethe N⁻ absorption layer, the window layer including the lowerconductivity layer, and an anode contact above the window layer. Theavalanche photodiode can include an N⁺ barrier layer below the N⁻absorption layer, an N⁺ conduction layer below the N⁺ barrier layer, asubstrate below the N⁺ conduction layer, and a cathode contact coupledwith the N⁺ conduction layer. The avalanche photodiode can include anavalanche region that is included in a mesa structure, the mesaextending into the N⁺ conduction layer so that a portion of the N⁺conduction layer is in the mesa and a portion forms a shoulder from themesa, with the cathode contact on the shoulder of the N⁺ conductionlayer. In one aspect, the avalanche photodiode can include: an oxidelayer above the window layer; a dielectric passivation layer coated ontothe window layer and over side surfaces of the mesa; an anode bond padcoupled with the anode contact; a cathode bond pad coupled with thecathode contact; and an environmental-protection passivation layer abovethe dielectric passivation layer, anode contact, and cathode contact,and the environmental-protection passivation layer having openings thatexpose the anode bond pad and cathode bond pad.

In one embodiment, an avalanche photodiode can include: an avalancheregion having one or more layers prepared from GaAs; a P⁻ absorptionlayer extending across the avalanche region; a P-type layer below acenter portion of the P⁻ absorption layer or the P-type layer extendingacross the mesa and below the entirety of the P⁻ absorption layer; andoptionally a lower conductivity region laterally from the P-type layerto a surface of the avalanche region and below a perimeter portion ofthe P⁻ absorption layer, the lower conductivity region having lowerconductivity compared to the P-type layer. The avalanche photodiode caninclude a window layer above the P⁻ absorption, the P⁻ absorption layerincluding the lower conductivity region, and an anode contact above thewindow layer. The avalanche photodiode can include an N⁺ barrier layerbelow the P⁻ absorption layer, an N⁺ conduction layer below the N⁺barrier layer, a substrate below the N⁺ conduction layer, and a cathodecontact coupled with the N⁺ conduction layer. In one aspect, theavalanche region is included in a mesa structure, the mesa extendinginto the N⁺ conduction layer so that a portion of the N⁺ conductionlayer is in the mesa and a portion forms a shoulder from the mesa, withthe cathode contact on the shoulder of the N⁺ conduction layer. In oneaspect, the avalanche photodiode can include: an oxide layer above thewindow layer; a dielectric passivation layer coated onto the windowlayer and over side surfaces of the mesa; an anode bond pad coupled withthe anode contact; a cathode bond pad coupled with the cathode contact;and an environmental-protection passivation layer above the dielectricpassivation layer, anode contact, and cathode contact, and theenvironmental-protection passivation layer having openings that exposethe anode bond pad and cathode bond pad.

In one embodiment, a method of manufacturing an avalanche photodiode caninclude: growing a base epitaxial structure having a substrate, aconduction layer above the substrate, a barrier layer above thesubstrate, an absorption layer above the substrate, and an active layerabove the absorption layer, absorption layer, and active layer havingGaAs with different doping; optionally processing the active layer toremove an annular perimeter region to leave a center active region; andgrowing a window layer over the center active region of the active layerand over the absorption layer. In one aspect, the active layer is anN-type active layer. In one aspect, the active layer is a P-type activelayer.

In one embodiment, the method includes forming the window layer to havea center stepped-up region above the center active region of the activelayer compared to a perimeter stepped-down region above the absorptionlayer, wherein the center stepped-up region and perimeter stepped-downregion include about the same thickness of material.

In one embodiment, a method of manufacturing an avalanche photodiode caninclude: growing a base epitaxial structure having a substrate, aconduction layer above the substrate, a barrier layer above thesubstrate, an absorption layer above the substrate, and an active layerabove the absorption layer, absorption layer, and active layer havingGaAs with different doping; optionally processing the active layer withimplantation to reduce conductivity of an annular perimeter region toleave a center active region; and growing a window layer over the centeractive region of the active layer and over the absorption layer. In oneaspect, the implantation is proton or beryllium implantation.

In one embodiment, any of the manufacture methods can include thefollowing: forming an oxide layer over the window layer; etching thebase epitaxial structure to form a mesa structure; depositing adielectric passivation layer over the mesa; etching the dielectricpassivation layer to form an anode contact area and a cathode contactarea; forming an anode contact in the anode contact area; forming acathode in the cathode contact area; forming an anode bond pad that iselectronically coupled with the anode contact area; forming a cathodebond pad that is electronically coupled with the cathode contact area;depositing an environmental-protection passivation layer; and removingportions of the environmental-protection passivation layer to expose theanode bond pad and cathode bond pad.

In one embodiment, the APD can be configured so that most or all of thelight coupled into the APD can be absorbed somewhere in the APDstructure so that none of the received light is reflected back out ofthe front surface of the APD. The anti-reflection surface on the frontof the APD can facilitate this process and inhibit light from beingreflected back through the optical fiber.

In one embodiment, in order to achieve a desired signal bandwidth of theAPD, it can be useful for most or all of the active layers between thetransparent P⁺ window layer and the transparent N⁺ blocking layer (e.g.,barrier layer) to be depleted so that the carriers generated by opticalabsorption and/or avalanche gain are moving at high velocity under theinfluence of the electric field in the depletion region. Increasinglyhigher signal bandwidth of the APD can be achieved by decreasing thetotal thickness of the active layers (e.g., most or all of the activelayers between the transparent P⁺ window layer and the transparent N⁺blocking layer). With a given material (e.g., GaAs) in the activelayers, the absorption coefficient can be substantially constant. Thus,as the active layers are made thinner for more bandwidth, reducedamounts of the incoming light may be absorbed in the active layers. Theavalanche gain can make up for the loss in quantum efficiency. However,if less of the incoming light is absorbed in the active layers, theresult can be more of the incoming light is absorbed in the N⁺conduction layer where it generates hole-electron pairs, which may beundesirable. Accordingly, one function of the N⁺ blocking layer can beto keep the minority carriers generated in the N⁺ conduction layer fromdiffusing into the active layers (e.g., together are the active region)and becoming a part of the signal response of the APD. As such, thisfunction can be a reason the N⁺ barrier layer can be referred to as ablocking layer. The N⁺ barrier layer can be made transparent (e.g., nooptical carrier generation) by using a material like AlGaAs that has ahigher bandgap than the GaAs active layers. Since the N⁺ barrier layerand N⁺ conduction layer are both N-type, the higher bandgap of the N⁺barrier layer also serves to keep the optically generated carriers inthe N⁺ conduction layer from diffusing into the active layers where theymay unfavorably decrease the signal bandwidth of the APD.

In one embodiment, the N⁺ barrier layer can be configured to provide twopurposes. First, the N⁺ barrier layer can be transparent at theoperating wavelength so that photon absorption does not generatecarriers in the N⁺ barrier layer. Second, the higher bandgap of the N⁺barrier layer can keep carriers generated by optical absorption in theN⁺ conduction layer from contributing to the electrical response of theAPD.

One skilled in the art will appreciate that, for this and otherprocesses and methods disclosed herein, the functions performed in theprocesses and methods may be implemented in differing order.Furthermore, the outlined steps and operations are only provided asexamples, and some of the steps and operations may be optional, combinedinto fewer steps and operations, or expanded into additional steps andoperations without detracting from the essence of the disclosedembodiments.

The present disclosure is not to be limited in terms of the particularembodiments described in this application, which are intended asillustrations of various aspects. Many modifications and variations canbe made without departing from its spirit and scope, as will be apparentto those skilled in the art. Functionally equivalent methods andapparatuses within the scope of the disclosure, in addition to thoseenumerated herein, will be apparent to those skilled in the art from theforegoing descriptions. Such modifications and variations are intendedto fall within the scope of the appended claims. The present disclosureis to be limited only by the terms of the appended claims, along withthe full scope of equivalents to which such claims are entitled. It isalso to be understood that the terminology used herein is for thepurpose of describing particular embodiments only, and is not intendedto be limiting.

TABLE 1 850 nm Target Spec Cumu- APD Thick- Limits lative Target SpecTarget Spec Target Spec Epi Design ness Thickness Thickness Xs Limitsn-doping Limits p-doping Limits Layer Name Material (nm) (nm) (nm) (—)Xs (cm⁻³) n-doping (cm⁻³) p-doping Substrate GaAs 0 — 0 — Marker AlGaAs50 40-60 50 0.3 0.26-0.34 Layer N- GaAs 3000 2700-3300 3050 0 1.00E+180.8-1.2E+18 Conduction Layer Ramp grade 50 40-60 3100 0->0.3 1.00E+18Lower AlGaAs 220 180-200 3320 0.3 0.26-0.34 1.00E+18 0.8-1.2E+18Cladding Layer Ramp grade 50 45-55 3370 0.3->0 5.00E+14 2.5-7.5E+14Spacer GaAs 5 3-7 3375 0 5.00E+14 2.5-7.5E+14 Layer N′ GaAs 17451700-1790 5120 0 3.00E+14 1.5E+13-1.174E+15 Absorption Layer N-Type GaAs355 335-377 5475 0 1.30E+17 1.22-1.38E+17 Layer Ramp grade 40 35-45 55150->0.3 1.3E+17->1.13E+19 Upper AlGaAs 852 801-903 6367 0.3 0.26-0.341.30E+19   1-1.6E+19 Cladding Layer Cap GaAs 40 30-50 6407 0 6.00E+195-7E+19

The invention claimed is:
 1. An avalanche photodiode comprising: anavalanche region having one or more layers prepared from GaAs an N⁻absorption layer extending across the avalanche region; an N-type layerabove at least a center portion of the N⁻ absorption layer; and a lowerconductivity layer laterally from the N-type layer to a surface of theavalanche region and above a perimeter portion of the N⁻ absorptionlayer, the lower conductivity layer having lower conductivity comparedto the N-type layer.
 2. The avalanche photodiode of claim 1, wherein theN-type layer extends across the N⁻ absorption layer.
 3. The avalanchephotodiode of claim 2, wherein the N-type layer extends across a mesahaving the N-type layer and the N⁻ absorption layer.
 4. The avalanchephotodiode of claim 1, wherein the lower conductivity layer having aconductivity reducing implant compared to conductivity of the N-typelayer.
 5. The avalanche photodiode of claim 4, wherein the implant is aproton implant or a beryllium implant.
 6. The avalanche photodiode ofclaim 1, comprising a window layer above the N-type layer, and an anodecontact above the window layer.
 7. The avalanche photodiode of claim 6,comprising an N⁺ barrier layer below the N⁻ absorption layer, an N⁺conduction layer below the N⁺ barrier layer, a substrate below the N⁺conduction layer, and a cathode contact coupled with the N⁺ conductionlayer.
 8. The avalanche photodiode of claim 7, wherein the avalancheregion is included in a mesa structure, the mesa extending into the N⁺conduction layer so that a portion of the N⁺ conduction layer is in themesa and a portion forms a shoulder from the mesa, with the cathodecontact on the shoulder of the N⁺ conduction layer.
 9. The avalanchephotodiode of claim 8, comprising: an oxide layer above the windowlayer; a dielectric passivation layer coated onto the window layer andover side surfaces of the mesa; an anode bond pad coupled with the anodecontact; a cathode bond pad coupled with the cathode contact; and anenvironmental-protection passivation layer above the dielectricpassivation layer, anode contact, and cathode contact, and theenvironmental-protection passivation layer having openings that exposethe anode bond pad and cathode bond pad.
 10. The avalanche photodiode ofclaim 1, comprising a window layer above the N-type layer and above theN⁻ absorption layer, the window layer including the lower conductivitylayer, and an anode contact above the window layer.
 11. The avalanchephotodiode of claim 10, comprising an N⁺ barrier layer below the N⁻absorption layer, an N⁺ conduction layer below the N⁺ barrier layer, asubstrate below the N⁺ conduction layer, and a cathode contact coupledwith the N⁺ conduction layer.
 12. The avalanche photodiode of claim 11,wherein the avalanche region is included in a mesa structure, the mesaextending into the N⁺ conduction layer so that a portion of the N⁺conduction layer is in the mesa and a portion forms a shoulder from themesa, with the cathode contact on the shoulder of the N⁺ conductionlayer.
 13. The avalanche photodiode of claim 12, comprising: an oxidelayer above the window layer; a dielectric passivation layer coated ontothe window layer and over side surfaces of the mesa; an anode bond padcoupled with the anode contact; a cathode bond pad coupled with thecathode contact; and an environmental-protection passivation layer abovethe dielectric passivation layer, anode contact, and cathode contact,and the environmental-protection passivation layer having openings thatexpose the anode bond pad and cathode bond pad.
 14. An avalanchephotodiode comprising: an avalanche region having one or more layersprepared from GaAs; a P⁻ absorption layer extending across the avalancheregion; an P-type layer below at least a center portion of the P⁻absorption layer; and a lower conductivity region laterally from theP-type layer to a surface of the avalanche region and below a perimeterportion of the P⁻ absorption layer, the lower conductivity region havinglower conductivity compared to the P-type layer.
 15. The avalanchephotodiode of claim 14, wherein the P⁻ absorption layer extends acrossthe P-type layer.
 16. The avalanche photodiode of claim 15, wherein theP-type layer extends across a mesa having the P-type layer and the P⁻absorption layer.
 17. The avalanche photodiode of claim 14, comprising awindow layer above the P⁻ absorption layer, the P⁻ absorption layerincluding the lower conductivity region and an anode contact above thewindow layer.
 18. The avalanche photodiode of claim 17, comprising an N⁺barrier layer below the P⁻ absorption layer, an N⁺ conduction layerbelow the N⁺ barrier layer, a substrate below the N⁺ conduction layer,and a cathode contact coupled with the N⁺ conduction layer.
 19. Theavalanche photodiode of claim 18, wherein the avalanche region isincluded in a mesa structure, the mesa extending into the N⁺ conductionlayer so that a portion of the N⁺ conduction layer is in the mesa and aportion forms a shoulder from the mesa, with the cathode contact on theshoulder of the N⁺ conduction layer.
 20. The avalanche photodiode ofclaim 19, comprising: an oxide layer above the window layer; adielectric passivation layer coated onto the window layer and over sidesurfaces of the mesa; an anode bond pad coupled with the anode contact;a cathode bond pad coupled with the cathode contact; and anenvironmental-protection passivation layer above the dielectricpassivation layer, anode contact, and cathode contact, and theenvironmental-protection passivation layer having openings that exposethe anode bond pad and cathode bond pad.
 21. An avalanche photodiodecomprising: an avalanche region having one or more layers prepared fromGaAs; an N⁻ absorption layer extending across the avalanche region; anN-type layer above at least a center portion of the N⁻ absorption layer;and an N⁺ barrier layer below the N⁻ absorption layer, an N⁺ conductionlayer below the N⁺ barrier layer, a substrate below the N⁺ conductionlayer, and a cathode contact coupled with the N⁺ conduction layer.